Generally, a ferrite stainless steel has characteristics such as excellent corrosion resistance, a low thermal expansion coefficient in comparison to an austenite stainless steel, excellent stress corrosion cracking resistance, and the like. Therefore, the ferrite stainless steel is widely used for dishes, kitchen utensils, exterior construction materials including roofing materials, materials for cold and hot water storage, and the like. Furthermore, in recent years, due to a steep increase in the price of Ni raw materials, the demand for replacing austenite stainless steels has been increasing; and therefore, the ferrite stainless steel has been used in a wider range of applications.
With regard to structures made of such a stainless steel, welding is an indispensable process. Originally, since the ferrite stainless steel had small solid solubility limits of C and N, the ferrite stainless steel had a problem in which sensitization occurred in welded portions and thus corrosion resistance was degraded. In order to solve the problem, a method has been suggested in which the amounts of C and N are reduced or a stabilization element such as Ti, Nb, or the like is added; and thereby, C and N are fixed so as to suppress sensitization in weld metal zones (for example, Patent Document 1), and this method has been widely put into practical use.
In addition, with regard to the corrosion resistance in welded portions of a ferrite stainless steel, it is known that the corrosion resistance is degraded in scale zones which are generated by heat input during welding; and therefore, it is important to sufficiently perform shielding with an inert gas in comparison to an austenite stainless steel.
Patent Document 2 discloses a technology in which Ti and Al are added at contents that fulfill the formula, P1=5Ti+20(Al−0.01)≧1.5 (Ti and Al in the formula indicate the contents of respective elements in a steel); and thereby, an Al oxide film that improves the corrosion resistance in weld heat-affected zones is formed in the surface layer of a steel during welding.
Patent Document 3 discloses a technology in which a certain amount or more of Si is added together with both of Al and Ti; and thereby, the crevice corrosion resistance in welded portions is improved
Patent Document 4 discloses a technology in which 4Al+Ti≦0.32 (Al and Ti in the formula indicate the contents of respective elements in a steel) is fulfilled; and thereby, heat input during welding is reduced so as to suppress the generation of scales in welded portions; and as a result, the corrosion resistance in welded portions is improved.
The above-described technologies in the related art aim to improve the corrosion resistance in the welded portions or the weld heat-affected zones.
In addition to the above technologies, as a technology to improve the weather resistance and the crevice corrosion resistance of a material itself instead of those of the welded portions, there is a technology in which P is added in a positive manner and appropriate amounts of Ca and Al are added (for example, Patent Document 5). In Patent Document 5, Ca and Al are added so as to control the shape and distribution of non-metallic inclusions in a steel. Here, the most peculiar point of Patent Document 5 is the addition of more than 0.04% of P, and there is no description of the effects during welding in Patent Document 5.
In a ferrite stainless steel in the related art, even when shielding conditions on welded portions are optimized, there are cases where black dots which are generally called as black spots or slag spots are scattered on weld back beads after welding. The black spot is formed by oxides of Al, Ti, Si, and Ca, which have a strong affinity to oxygen, solidified on a weld metal during the weld metal is solidified in a tungsten inert gas (TIG) welding. The generation of black spots is greatly affected by welding conditions, particularly, the shielding conditions of an inert gas, and the more insufficient the shielding is, the more black spots are generated.
Here, since the black spot is an oxide, there is no problem on the corrosion resistance and the formability of welded portions even when a small number of black spots are scattered. However, if a large number of black spots are generated or black spots are generated continuously, the appearance of welded portions is impaired in the case where the welded portions are used without being polished, and in addition, there are cases where black spot portions are separated when the welded portions are processed. In the case where the black spot portions are separated, there are cases where problems occur in which the formability is degraded, and crevice corrosion occurs in gaps between the separated black spot parts. In addition, even when no process is performed after welding, in the case where thick black spots are generated in products in which a stress is applied to welded portions because of its structure, there are cases where the black spots are separated; and thereby, the corrosion resistance is degraded.
As a result, in order to improve the corrosion resistance of TIG welded portions, it is important not only to simply improve corrosion resistance of weld bead zones and weld scale zones, but also to control black spots that are generated in the welded portions. With regard to scales involving discoloration which occurs during welding, it is possible to suppress the majority of the scales by a method in which shielding conditions of welding are enhanced. However, with regard to black spots generated in TIG welded portions, in the related art, it is not possible to sufficiently suppress the black spots even when the shielding conditions are enhanced.